Electrical resistance element



Patented Mar. 13, 1945 ELECTRICAL RESISTANCE ELEMENT RobertRosc Barrington, Luton,England, assignor to General Motors Corporation, Detroit, Mlcln, a corporation of Delaware Application May 4,1942, Serial No. 441,617 InGreat Britain July 10, 1941 13 Claims.

This invention relates to electrical resistance elements for use in radio and similar electrical installations.

The object of the invention is to achieve stability of operation and simplicity of manufacture of electrical resistance elements.

Such elements. made from a mixture of comminuted conductive and non-conductive materials are apt to be unstable in operation, for with currents in excess of certain values, the normal resistance value changes. This is accentuated under high temperature conditions.

It is thought that the reason for this instability is that the small areas of contact between particle and particle, and which in total produce the resistance effect, become contaminated or change their chemical and physical characteristics. On this basis, the problem resolves itself into maintaining constancy of operation of these minute areas.

The present invention achieves this by the use of acomminuted electrically conductive refractory material such as silicon carbide, coated with such an. amount of a metallic conductor as will prevent the breakdown of the small points of contact but will not reduce the resistance value to normal conductivity.

Finely divided copper is the preferred metallic conductor. The proportion of this latter should be between 5% and 20% of the whole mixture. The lower value should be suillcient to prevent a gradual increase in resistanceunder operatin: conditions; th higher value should be suiilcient to prevent an increase in conductivity. A bondinfl element of the glassy type may be found necessary in most circumstances.

In one example of carrying the invention into eilect. 18% of very iinely divided copper is mixed with 18% 0! equally finely divided dehydrated boric acid. These are ground together and then mixed with 64% oi finely divided silicon carbide. The mass is then compacted to the desired shape or placed in situ where desired, and finally heated to about 900? C. The minture so formed and treated should be sealed iron; the atmosphere to prevent oxidation or the metal.

termed in thaibore oi the insulator.

, Inso applying the presentinvention to spark Suit able metallic contacts can be provided on the.

plugs, the resistance element is sealed in place by a. glass seal, preferably at the top and bottom by a gas-tight electrically conductive seal.

To illustrate this, one form of an assembled spark plug is shown in section in the accompanying drawing. Within the outer shell 0, which carries a screening connector 2; adapted to be attached to the cable harness (not shown), is the insulator c. Before being assembled in the shell as shown, it is first fired to the necessary denseness in the usual way, and then the inner electrode assembly is made up as follows.

The insulator bore contains at its lower end an electrode d which is loosely held in place by its upper enlarged end co-operating with the bore .of the insulator or by any other equivalent means. If desired it may be cast in place. On to the top of the electrode d so positioned is placed a small pellet of a ground mixture e of powdered copper (55%) and a borosilicate glass (45%) bonded with dextrine. .A small copper disc I is then pressed down on to the pellet, and on top of that is placed a mixture 1 of powdered silicon carbide (64%), powdered copper (18%), h d ated boric acid (18%) mixed in the manner above described. A further copper disc h is then pressed on this resistance mixture, and then on top'of this disc is placed a. further pellet t of a ground mixture of copper powder (50%) and a low-melting-point glass (50%). The whole mass isthen compressed, by the conductive, threaded centre-wire j if desired, and heated at a temperature of about 900 C. Thus the same heat treatment melts the glass seal, and at the same time matures the resistance material. While in the soft heated condition, the mass is again pressed so as to obtain a proper gas-tight seal. After cooling the insulator assembly is inserted in the shell a. as shown in the drawing.

This procedure can be varied: for example, if desired the resistance element can be preformed and sealed, and more than one such element may be combined serially with several glass seals. Such and similar modifications are to be deemed within the invention. It will be appreciated too, that the exact forms of the lower electrode part d and of the upper electrode part i, are immaterial so long as they allow the desired sealing to be effected.

While silicon carbide is the preferred retractory conductive material other carbides such as those of boron and tungsten can be used, al-

though it is thoughtthat the reliability of perof the metallic conductor being, under normal operating conditions, sufficient to prevent a grad- .ual increase of resistance and insuflicient to cause an increase in conductivity, and heating the mixture to set the bonding element.

2. A resistance element comprising a mixture of finely divided silicon carbide, finely divided copper in an amount between 5% and of the whole, and a hardened glassy bonding element.

3. A resistance element comprising a mixture of finel divided Silicon carbide, finely divided copper in an amount between 5% and 20% of the whole, and fused boric acid as a bonding element.

4. A resistance element comprising about 64% of finely divided silicon carbide, about 18% of finely divided copper, and the remainder a hardened bond of the glassy type.

5. A resistance element comprising about 64% of finely divided silicon carbide, about 18% of finely divided copper, and the remainder fused boric acid as a bond.

. 6. A sparkgplug having an insulator, and in said insulator, a central bore, a lower electrode and an upper electrode being positioned in said bore, and between said electrodes a resistance element comprising a stable mixture of finely divided silicon carbide and copper bonded together by a glassy bond interposed between a conductive glass-seal on the top of the upper end oi. the

' lower electrode and a conductive glass seal below the lower end of the upper electrode.-

7. A spark plug comprising an insulator, a central bore, a lower electrode and an upper electrode in said bore and between said electrodes a resistance element comprisin a stable mixture of'finely divided silicon carbide and copper to an amount between 5% and 20% bonded together by a glassy bond interposed between a conductive glass seal on the top of the upper end oi' the lower electrode and a conductive glass seal below the lower end of the upper electrode.

- 8. A spark plug comprising an insulator, having a central bore, a lower electrode and an upper electrode in said bore and between said electrodes a resistance element comprising a stable mixture of about 64% finely divided silicon carbide and 18% finely divided copper bonded together byfused borls acid, interposed between a conductiveglass seal on the'top of the upper end of mixture of finely divided silicon carbide and copper bonded together by'a glassy Mod, a second metallic disc, and a second conductive glass seal.

10. A spark plug comprising an insulator having acentral bore, a lower electrode and an upper electrode in said bore, andbetween said electrodes in series from the top end of said lower electrode, a conductive glass seal, a metallic disc, a stable mixture of about 64% finely divided silicon carbide and about 18% of finely divided copper bonded together by a glassy bond, 9. second metallic disc, and a. second conductive glass seal.

11. The method of making a spark plug assembly including an insulator having a central bore which comprises positioning a lower electrode in said bore and inserting in series on top of the upper end of said electrode, a conductive glass seal, a metallic disc, a mixture of finely divided silicon carbide and copper and dehydrated boric acid, a second metallic disc, and a second conductive glass seal, and then heating the assembly to about 900 C. to soften the inserted ma-. terials, inserting an upper electrode into contact with the upper seal, and then allowing the assembly to cool.

12. The method of making a spark plug assembly including an insulator having a central bore which comprises positioning a lower electrode in said bore and inserting in series on top of the upper end of said electrode, a conductive glass seal, a mixture of finely divided silicon carbide and between 5% and 20% of finely divided copper and dehydrated boric acid and a second conductive glass seal, and then heating the assembly to about 900 C. to soften the inserted materials, inserting an upper electrode into contact with the upper seal, and then allowing the assembly to cool.

13. The method of making a spark plug assembly including an insulator having a central bore which comprises positioning a lower electrode in said bore and inserting in series on top of the upper end of said electrode, a conductive glass seal, a mixture of finely divided silicon carhide and copper and a glassy bond, and a second conductive glass-seal, and then heating the assembly to about 900 C. to soften the inserted materials, inserting an upper electrode into contact with the upper seal, and then allowing the assembly to cool. k

ROBERT ROSS BARRINGTON. 

